The heat exchanger should maintain a good heat transfer performance when a medium flows through it which has a strong tendency to deposit a coating on the channel walls. In the following description this medium is called "the primary medium" or "the process medium". The primary medium may be a product flow from a process in the form of a gas with solid particles, flue gas with soot, or a liquid. On the other side of the heat transfer walls flows a second medium, called "the secondary medium" or "the service medium", whose task is either to cool or heat up the primary medium. The secondary medium may be a gas or a liquid.
The helical insert has internal channels through which the secondary medium flows. The cross section of the insert may be in the form of one or more rectangular tubes adjacent to one another or several round tubes adjacent to one another, and for the sake of simplicity is called "tube spool" in the following description.
At one end of the cylindrical housing there is an intake for the primary medium, which flows through the windings in the insert or inserts to the outlet at the other end. The secondary medium can be parallel flow or counterflow according to what is most suitable for the process.
The invention comprises a heat exchanger which is equipped with a central tube which extends along the centre axis of the housing. The central tube is both axially movable and rotatable. On the central tube there is mounted a device for removal of deposits on the walls of the channel in which the primary medium is conveyed.
On the heat transfer surfaces of a heat exchanger particles will often be precipitated which will adhere to the surfaces and be deposited as a coating which will reduce the heat transfer. The performance of the heat exchanger is highly dependent on its having clean surfaces. It has been shown that even a thin layer of particles or a thin coating of deposits will substantially reduce the performance. If a thicker layer of coating is formed it will also narrow the channel opening, thus increasing the flow resistance and thereby obstructing the through-flow of the medium.
The temperature of the primary medium is sometimes so high that the coating hardens after a short time and it thus becomes necessary to keep the cooling surfaces clean in an efficient manner without the addition of foreign matter which will pollute the product flow.
A common problem with heat exchangers is that it is a relatively complicated process to remove fouling. Many different designs of cleaning equipment are known and many methods for internal and external removal of fouling on tubes, plates, shell and housing.
The usual method of cleaning heat exchangers is to wash both the tubes and the housing with a liquid to which may be added a solvent for the fouling concerned. Another method which is used is to dismantle the entire heat exchanger and clean the whole tube bundle and housing mechanically by means of washing and brushing. However, both of these methods require the heat exchanger to be disconnected from the process, which is normally both a costly and laborious procedure.
In WO 88/01362 there is disclosed a heat exchanger with a plurality of helical tube spools wherein the tube spools are composed of a plurality of parallel tubes located beside one another. The tube spools with a distributing head at each end are mounted on to a longitudinal central tube, thus enabling the entire tube bundle with the distributing heads to be withdrawn from the housing. The dismantling process is thereby facilitated, thus reducing the cleaning time. However, the heat exchanger is not designed to be self-cleaning or with cleaning equipment.
In NO 45071 there is disclosed a rotating heat exchanger with permanently installed scraper devices. The scraper devices are located in the channels in which the flue gas is conveyed and will scrape off soot on the cooled surfaces. The scraper devices, however, cover the entire channel cross section, thus making it necessary to direct the flue gas on both sides of the devices.